Crystallinity dependence of high-temperature oxidation of silicoboron carbonitride monoliths

Abstract Comparative investigation of performances of metastable materials in amorphous and crystalline states is one of the keys to exploring their applications in engineering. The oxidation behavior and kinetics of the partially crystalline Si2BC3N (PCS) monoliths, i.e. amorphous Si2BC3N with precipitation of nanocrystallites (10-30 nm) of SiC and BN(C), were investigated at 1500-1700 °C with dwell time ranging from 0.5 h to 16 h. Oxidation leads to the formation of amorphous SiO2 and cristobalite, and more sufficient crystallization of PCS. Due to the release of gases including CO, CO2, N2 and evaporation of B2O3, PCS undergoes weigh loss at 1500-1700 °C. Loose oxide scales with low adhesion strength are formed at 1500-1600 °C, while the scales are highly dense, adherent at 1700 °C indicating good oxidation resistance of PCS. Compared to completely amorphous Si2BC3N (CAS), PCS exhibits lower activation energy of ∼50.0 kJ‧mol-1 for 1500-1600 °C/≤16 h, and higher parabolic rate constants (42.5 μm2‧h-1 for 1500 °C/≤16 h, 172.2 μm2‧h-1 for 1600 °C/≤16 h, and 120.4 μm2‧h-1 for 1700 °C/≤4 h), which suggests the inferior oxidation resistance of PCS. One most possible reason is that the changes in superficial chemical compositions and reactivity during crystallization cause larger oxygen uptake on PCS surfaces.